At the present day, there is known a so-called radio-controlled timepiece which receives long-wave standard time radio waves with time data, i.e., a time code transmitted in respective countries (e.g., Germany, United Kingdom, Japan and others) and corrects time data of a clocking circuit based on the received radio waves.
Meanwhile, when radio waves are received by using, e.g., a bar antenna, there is used a tuning circuit which receives radio waves by causing an inductance of the antenna, a capacitor and the like to be resonant with radio waves having a desired frequency. In such a tuning circuit, tuning with radio waves having a desired frequency is effected by changing a capacity to be connected to the antenna.
It is general that a conventional radio-controlled timepiece comprises a radio wave receiving circuit including such a tuning circuit. As one of such circuits, there is known a radio wave receiving circuit which performs tuning with long-wave standard time radio waves by using a method to attach a plurality of chip capacitors.
That is, at the time of industrial assembly, an inductance of an antenna is measured, and chip capacitors whose capacitances are not more than a desired capacitance are first attached by soldering. Then, a resonance frequency is measured, an insufficient capacitance is calculated, and a chip capacitor whose capacitance is slightly smaller than the insufficient capacitance is further attached by soldering. Furthermore, operations to measure the resonance frequency and adjust the tuning capacitance are repeatedly carried out according to needs, and adjustment is effected in such a manner that the radio wave receiving circuit performs optimum tuning with respect to the long-wave standard time radio waves.
Moreover, Japanese Patent Application KOKAI Publication No. 6-125280 discloses a radio wave receiving circuit which comprises two capacitors included in a tuning circuit in parallel and can select a resonance frequency by switching a connection of one of the capacitors based on ON/OFF of a switch and changing a tuning capacitance. However, this is used to switch a resonance frequency to be selected, but it is not intended to change a capacitance in order to tune with radio waves having a desired frequency.
The method for attaching the plurality of chip capacitors requires adjustment of a tuning capacitance when assembling a product, but operations to measure a resonance frequency and attach capacitors must be repeatedly carried out in that adjustment. Therefore, the number of working steps, a working time, a cost and others are taken. Additionally, capacitors, a switch element which switches the capacitors and others are required in accordance with the number of frequencies of radio waves to be received. Therefore, when receiving a plurality of radio waves, applying the technique disclosed in Japanese Patent Application KOKAI Publication No. 6-125280 increases the number of components or a substrate area, and hence a reduction in size of the circuit is difficult.
Further, if tuning adjustment is performed only in a circuit substrate for tuning having an antenna and capacitors mounted thereon and then the adjusted circuit substrate is set in a radio-controlled timepiece and connected with a timepiece circuit substrate, a resonance frequency deviates due to an IC other than the tuning circuit substrate, an input capacitance of the timepiece circuit substrate or the like. Therefore, when trying to perform complete adjustment of the tuning capacitance, tuning adjustment must be again performed in the entire radio-controlled timepiece.